The present invention relates to a method and apparatus for determining the content of fluid components, in particular of milk components, based on the generation of acoustical and electrical fields and the measurement of acoustical and electrical characteristics of the sample. More particularly the present invention can be used to determine the content of fat, protein, lactose, mineral salts and water and optionally additional components of milk, by the determination of the acoustical and electrical parameters of a milk sample at different temperatures.
There are many applications where it is desirable to determine various characteristics of a fluid, such as the concentration of material in a solution, suspension or emulsion. An example of such emulsion is milk. Milk contains globules of butterfat (2-6%), as well as 2-6% proteins (mostly casein), 4-5% milk sugar (lactose), and 0.5-1% mineral salts dispersed in an aqueous solution. The proteins, lactose and mineral salts, taken together, comprise the solid-not-fat (SNF) component. Water constitutes 80-90% of the whole milk [Bhatti, S. S. et al. Acustica, Vol. 62, p. 96-99, 1986]. The determination of the chemical components in milk and milk products is of considerable importance in the dairy industry, since the amount of such components, particularly the butterfat and protein contents thereof, is the usual basis for determining the products' price, food value and compliance with US state and federal laws and regulations.
Various attempts have been made to determine the content of milk components using different physical methods. U.S. Pat. No. 5,033,852 discloses an optical measuring device for measuring the fat content of milk. The optical measurement device comprises a light source, an optical condenser, a glass test tube provided for holding a set amount of the milk to be tested, a tubular housing for the milk test tube; a lens and a photosensitive element, and a digital display. The milk fat content is obtained by automatically measuring, using the photosensitive element, the diffusion depth of the light projected on a milk sample. The device provides quick, accurate and reliable measurement results. However the disclosed method is limited in its application to determining the fat content, and other milk component cannot be analyzed.
U.S. Pat. No. 5,983,709 describes a device and method for measuring, monitoring, and controlling the fat and protein (micellar associated protein) content of a milk sample. The invention is especially adaptable for the standardization of milk for cheese production. The device consists of a sampling inlet feeding a milk sample into a diluter. The diluter has the ability to select an appropriate diluent for dilution that renders the milk sample analyzable by a spectrophotometer. For fat determination, the milk sample is diluted with a detergent-chelating agent that breaks up micellar protein. Light absorbance readings on the spectrophotometer of chelator-detergent diluted milk samples are concentration-dependent only on the fat content. Dilution of the milk sample with water gives absorbance readings that are dependent on the concentration of fat plus micelle-associated proteins. The micelle-associated protein concentration can then be obtained by subtracting the fat concentration from the total. The method described involves many steps, protein and fat determination are carried out in different samples and the protein determination is indirect since it is based on the subtraction of the fat concentration of the sample from the fat-plus-protein concentration. Since the fat and fat-plus-protein contents are measured by light absorbance, only fat and micelle-associated proteins such as caseins can be determined, while other milk components such as salt and lactose cannot be analyzed.
U.S. Pat. No. 4,566,312 describes apparatus and processes for automatic determinations of fat contents of foods, such as diary products, e.g. milk, wherein automatic density and solids content determining apparatus are employed, together with a computer, to determine the fat content of food being tested. The density determining apparatus is preferably one that is electromagnetically excited to vibrate at its natural resonant frequency, so that the mass of the sample may be determined from the change in such frequency, in comparison with a control. The means for measuring the solids content include electromagnetic radiation (microwave energy), which is employed to drive off the volatile material (usually mostly water) in the sample, which is automatically weighed before and after such volatilization. The fat content is calculated from the data on the density and the solids content. By means of this invention the fat contents of diary products, as well as contents of certain comparable components of other materials can be rapidly and accurately determined from small samples of such materials, thereby facilitating rapid evaluations, production controls and standardization of such materials leading to an important saving of time and money. However the apparatus described is applicable only for fat determination, while other milk components cannot be analyzed.
U.S. Pat. No. 4,447,725 describes an improved electro-optical apparatus for measurement of fat, protein, lactose and solids in milk. The invention also discloses an improved method and apparatus for quantitatively measuring fat concentration in fat emulsions such as synthetic and natural dairy products, including specifically milk, which minimizes errors and inconsistencies due to variations in fat molecular weight or degree of lipolysis. The method is based on irradiation of a milk sample with energy in the infrared spectrum at a wavelength characteristic of different linkages, groups and bonds. The apparatus described cannot be used to measure milk samples (e.g. taken from a cow) on-line, and requires transferring samples to a laboratory, which lengthens the time of analysis.
It is widely recognized that there is a lack of suitable sensors for providing information about the physicochemical properties of foods, especially for the continuous monitoring of foods (milk, wine, juices and others) during processing. One of the major problems in developing analytical techniques for use in the food industry is the diversity and complexity (both compositional and structural) of liquid food products. Many traditional “wet-chemistry” techniques have limited application because they are destructive, time consuming, and labor intense. Consequently there has been a drive to develop new technologies, or to apply techniques currently used in other areas, for analysis of liquid food products.
Over the last decade there has been increasing interest in the use of ultrasound for characterizing food materials. In this technique, a high-frequency sound wave is propagated through the material being tested. Information about the properties of a material is then obtained by measuring the type and degree of interaction between the sound wave and the material. Ultrasound has major advantages over many other analytical methods because it is nondestructive, rapid, precise, relatively inexpensive, and can be applied to concentrated and optically opaque samples [McClements, D. J., Critical Reviews in Food Science and Nutrition, Vol. 37, p. 1-46, 1997].
As known in the art, e.g. as exemplified by the teachings of U.S. Pat. No. 3,040,562, the ultrasound velocity propagation in whole milk varies with the amount of fat present in the milk and with the sample temperature. U.S. Pat. No. 4,145,450 discloses a method and apparatus for controlling the butterfat content of a stream of milk on a continuous basis. The innovation of the method disclosed is the determination of sound velocity at two different temperatures, preferably 45° C. and 65° C., and the application of an equation system from which high concentrations of milk components can be determined.
U.S. Pat. No. 4,145,450 shows that the ultrasound velocity, U, in the milk is directly correlated with the percentage of butterfat (%F) and the percentage of solids-not-fat (%SNF) in the milk, with the relationship between U, %F and %SNF dependent upon the particular temperature of the milk. Consequently, by measuring ultrasound velocity at two temperatures, e.g. at 45° C. and 65° C., it is possible to evaluate the fat content in the milk. Thus, the following simultaneous linear equations could be solved:U45=K1(%F)+K2(%SNF)+K3U65=K4(%F)+K5(%SNF)+K6where U45 and U65 are the ultrasound velocity at respectively 45° C. and 65° C., and K1-K6 are parameters determined empirically by correlation with a separate analysis performed by known chemical methods, K1-K6 hold substantially constant for butterfat levels ranging between 0 and 20%. For a wider range of fat content, second order equations are needed. The method described in U.S. Pat. No. 4,145,450 is limited in its application for analyzing the butterfat and solid-not-fat content of milk, while constituents of the solid-not-fat itself (i.e. protein, lactose and salts) and other milk components cannot be determined.
Since the quality and price of the milk product is determined according to the percent of fat, protein and somatic cells, previous methods that determine only %F and %SNF are not adequate.
There is thus a widely recognized need for, and it would be highly advantageous to have, an apparatus and method for the simultaneous, on-line determination, of the content of at least three components of a fluid, in particular milk, rapidly and accurately, using small samples of the fluid.